Chemical sensors and, especially biosensors operating with bioactive components are known and are the subject of vigorous development.
Such sensors basically include a surface layer with a sensor-active material which is exposed to the medium to be tested, especially a liquid. This layer contains the sensor-active material which can be immobilized, for example, in a membrane composed of polyvinylchloride. The signals supplied by the sensor-active material under the effect of an analyte are transformed by a transducer element and are acquired in registrable form by electronic signal processing which can be provided by integrated electronics.
As transducers, particularly semiconductive electrodes, field-effect transistors, potentiometric and amperometric electrodes and the like are considered.
All known sensors have the common characteristic that they generally have insufficient adhesion of the sensor membrane on the respective base element, i.e. the measurement solution can wash it away or penetrate in a detrimental manner directly into the sensor membrane and damage it irreversibly. Connected therewith are stability problems and drift problems with respect to the sensor output signals. Furthermore, there is the need to establish the out-diffusion, i.e. the bleeding or washing out, of sensitive membrane components into the solution. Such sensors, as a consequence have only a limited life. It is also a drawback that the contacts and signal electronics are only separated by a thin passivation layer from the sensitive region and thus the sensor is highly prone to damage.
An attempt to solve this problem is described by Knoll in DE 41 15 414 A1. Here, anisotropic etch pits are incorporated in the substrate material as containments in which the sensor membrane can be anchored. The containments can be jacketed in a nonconductive material and additionally, conductive electrodes can be deposited. These vertical containments comprise openings converging toward the rear side of the chips which are in direct contact with the measurement solution. As a consequence, the above-described drawbacks are overcome. Especially so-called sensor arrays can be realized with this technique with different sensitivities to different substances or ion types.
With this kind of sensor, the containment formation affords a certain protection of the ion selective membrane to limit bleeding out and dissolution. This technology is however only apparently simple in that it can be realized only with expensive lithography techniques.